1. Field of the Invention
The present invention relates generally to an air/fuel ratio control system for an internal combustion engine. More specifically, the invention relates to an air/fuel ratio control system which can precisely derive a fuel delivery amount correction value.
2. Description of the Background Art
In general, air/fuel ratio control in an automotive internal combustion engine is performed by monitoring oxygen concentration in an exhaust gas from the engine and by feedback controlling the fuel delivery amount so that the air/fuel ratio in an air/fuel mixture to be introduced into the engine combustion chamber is maintained at or near an optimal or a stoichiometric value. A correction coefficient is typically derived from a proportional component and an integral component. The fuel delivery amount is basically derived on the basis of engine speed and and engine load and then corrected by various correction values respectively derived depending upon associated correction parameters. In the case of a fuel injection type internal combustion engine, the basic fuel injection amount Tp is typically expressed by: EQU Tp=k.times.Q/N
where
k is constant; PA1 is engine load, i.e. intake air flow rate; PA1 N is engine speed.
The basic fuel injection amount Tp is corrected by a correction coefficient K.sub.COEF which is a combination of a variety of correction coefficients, such as an acceleration enrichment correction coefficient, a cold engine enrichment correction coefficient and so forth, an air/fuel ratio dependent correction coefficient K.sub..lambda., a battery voltage compensating correction value Ts and so forth. Correction of the basic fuel injection amount Tp utilizing these correction values is per se well known technology in the art. The corrected fuel injection amount is used as a fuel injection amount Ti to be actually injected.
As set forth, the air/fuel ratio control by adjusting the fuel delivery amount is performed in a feedback manner depending upon the oxygen concentration in the exhaust gas and utilizing proportional-integral PI control strategy. In conventional air/fuel ratio control, the proportional component P is derived on the basis of an oxygen sensor signal level varying across a threshold level corresponding to the stoichiometric value of the air/fuel ratio. Therefore, the proportional component P is swiftly varied when the oxygen sensor signal level varies across the threhold level. According to a swift change of the proportional component P, the air/fuel ratio dependent correction coefficient K.sub..lambda. varies at a significant level. Then the air/fuel ratio dependent correction coefficient K.sub..lambda. is moderately increased or decreased at a gradient defined by the integral component I. Typically, the integral component I is derived by multiplying a basic integral component i which is derived by looking up a table in terms of the engine driving condition, by the fuel injection amount Ti.
As is set forth above, the fuel injection amount Ti is derived with various correction coefficients. Therefore, the integral component is influenced by the correction coefficients for making the air/fuel ratio dependent correction coefficient not precisely corresponding to the engine driving condition. For example, in the low engine load condition, the battery voltage compensation value Ts may be significantly influenced to make the integral component I excessively large to degrade exhaust control characteristics.
On the other hand, according to recent trends, the requirement for anti-polution control is becoming more and more strict for reducing CO, NO.sub.x and other pollutants. To comply with such a requirement, a mixture of gasoline and alcohol is interesting as a new automotive fuel for a lower exhaust rate of pollutants. On the other hand, because of the low combustibility of alcohol, the air/fuel mixture ratio has to be differentiated from that of the air/gasoline mixture ratio. Since the optimal air/fuel ratio is variable depending upon the mixture ratio of alcohol versus gasoline, correction of the fuel injection amount depending upon the alcohol/gasoline mixture ratio has to be made for obtaining optimal engine performance.
In such a case, the alcohol/gasoline mixture ratio dependent correction coefficient may provide a substantial influence on the integral component I. Therefore, the air/fuel ratio dependent correction coefficient K.sub..lambda. varies significantly when the alcohol/gasoline mixture ratio is varied.